We are analyzing the structure of key fibrous protein assemblies to understand their functional roles. Muscle is a major system under study. X-ray diffraction and electron microscopic studies are being carried out on the tropomyosin-troponin complex, which is the regulatory switch on the thin filaments in muscle; and on molluscan myosin, where the regulatory switch is associated with the enzymic portion or "head" of the molecule. The proteins are being examined in the isolated states, in crystalline arrays and in whole muscle. We are also determining the structure of microtubules by analysis of x-ray fiber diagrams. A further goal of this project is the x-ray crystallographic study of modified fibrinogen, to obtain a low resolution image of the molecule, and to identify its interactions in the assembly of the fibrin clot. BIBLIOGRAPHIC REFERENCES: Cohen, C. "Protein Assemblies and Cell Form." Trends in Biochem. Sci. 2, #3, 51-55 (1977). Tooney, N.M. and Cohen, C. "Crystalline States of a Modified Fibrinogen." J. Mol. Biol. 110, 363-385 (1977).